Apparatus for slip-casting ceramic materials



July 3, 19.62 w. A. DANIEL 3,041,699

APPARATUS FOR SLIP-CASTING CERAMIC MATERIALS Original Filed Sept. 23, 1955 2 Sheets-Sheet 1 j.; i /5 44 r z 27 26 28 ,8

l/ 40\1 l Y /gg /3 2/ ATTORNEY July 3, 1962 w. A. DANIEL 3,041,699

APPARATUS FOR SLIP-CASTING CERAMIC MATERIALS Original Filed Sept. 23, 1955 2 Sheets-Sheet 2 INVENTOR I4. A. Dan/'e/ Q/ZM ATTO NEY t cnam. (ci. 2s-29) This invention relates to the art of slip-casting ceramic objects in water-absorbent molds, and particularly to that phase of the art involving the centrifugal casting of hollow ceramic cylinders. This application is a division of my application Serial No. 536,155, tiled September 23, 1955, now Patent No. 2,962,790.

An object of the invention is to slip-cast ceramic cylinders of high quality that retain their shape during subsequent drying and ring steps.

Another object is to provide a practicable apparatus for centrifugal slip-casting ceramic cylinders that reduces the number of imperfect cylinders (rejects) below that common with prior apparatus.

A feature of the invention is the inclusion in the apparatus of means allowing the determination of completion of the optimum duration of the spinning time to produce a good casting.

Another feature is an apparatus that produces uniform distribution of slip of uniform characteristics in a centrifugal mold.

Other more speciiic objects and features of the invention will appear from the description to follow.

Although the invention is useful in the centrifugal slip-casting of ceramic bodies of various compositions and for various purposes, it will be discussed in connection with the production of ceramic cylinders or rings of materials such as barium titanate for use as electroacoustic transducers. Considerable ditiiculty has been encountered in the manufacture of relatively large cylinders of this material.

The best previously known method, known as slipcasting, was to introduce the slipta liquid suspension of the desired ceramic material in nely comminuted form) into a cylindrical plaster of Paris mold and rotate the mold about its axis to first distribute and then retain the slip (by centrifugal force) in a uniform layer against the cylindrical wall of the mold until it became stiif enough, by absorption of water therefrom by the plaster mold, to retain its shape, after which the mold was stood on end to complete the drying at a slow rate. This procedure produced many defective castings (rejects). A large proportion of the castings either distorted as a result of too short a spin period to set the slip, or cracked because of tovo long a spin period. I have discovered that cracking results from over-spinning, because the centrifugal force urging the water out of the slip dries the inner portion of the slip layer at an excessive rate relative to the outer portion. I have further discovered that the optimum spinning period is very critical, and that the minimum number of rejects results when the spinning is stopped just as the last free water leaves the inner surface of the casting. In, this connection, it should beV understod that during the spinning operation some free liquid rst migrates to the inner surface of the casting by centrifugal action, because the liquid is less dense than the suspended solid particles, but after the latter n have been compacted against the mold, the free water at the inner surface migrates through the solid particles and into the plaster mold by capillary attraction and centrifugal force. If the centrifugal force is terminated just as the free liquid leaves the inner surface, further outward migration of'liquid continues solely by capillary 3,0416@ Patented July 3, 1962 attraction of the plaster mold, which tends to dry most rapidly that portion of the casting next adjacent to the mold. In other words, the absorption of water by the plaster mold produces a gradient from minimum moisture content at the outer surface of the casting in contact with the mold to maximum moisture at the inner, free surface of the casting. This tends to prevent cracking.

Although the present invention resides in part in the new concept that the optimum spinning period is that just sumcient to dissipate freeliquid from the inner surface of the casting, an important feature is the provision of apparatus whereby the time of disappearance of the free liquid can be determined. In previously known centrifugal casting apparatus, it was impossible to observe the casting during the spinning operation, and I have found it impractical to determinein advance what the spinning time must be. In accordance with the invention, I provide a transparent end wall for the cylindrical plaster mold through which the inner surface and also the end surface of the casting that is against the transparent end wall can be observed during the spinning.

The visible change in the casting as the spinning proceeds will be described later with reference to the drawing.

A full understanding of the invention may be had from the following detaileddescription with reference ,to the drawing, in which:

FIG. 1 is a front elevation view of a complete apparatusfor practicing the invention.

FIG. 2 is a longitudinal sectional view through a portion of the apparatus in FIG. 1.

FIG. 3 is a diagram showing a method of formingan end cap on a cast cylinder.

FIG. 4 is a diagram showing the position of a casting and mold for iinal drying. j

FIGS. 5, 5a, 5b and 5c are diagrams showing the appearance of the end of a casting through the transparent end wall of the mold during various stages of the spinning operation.

Referring to FIG. 1, there is shown a portion of a conventionalv lathe 1t) having a chuck 11 Vadapted to be rotated by a drive mechanism (not shown) and a tail stock 12.

Rotatably supported by the chuck 11 and the tail stock 12 is a mold assembly having an arbor 13, one end of which is engaged by the chuck 11 and the other by the tail stock 12. There is mounted on the lathe a motor 14 driving a pump 15 which delivers slip from a funnel 16 through a flexible tube 17 to a iitting 18.

Referring to FIG. 2, the arbor 13 is hollow intermediate its ends to provide -a passage for delivering slip from the connection 18 into the mold. The nipple 18 connects to a hollow bushing 19 encircling the shaft and sealed with respect to the external surface thereof by a pair of packing glands 2t) in the form of washers of resilient material which are compressed between the opposite ends of the bushing 19 and a pair ofball thrust bearings 21, 21. Lettward movement of the cone of the left bearing 21 is prevented by a shoulder 22 on the arbor, and the coneof the right bearing 21' is adjusted toward the shoulder 22, by a nut 24 which is threaded onto-the arbor. The arbor is provided with3oneor more radial holes 25 communicating the space within the bushing 19 with the central passage-of the arbor.

The mold proper comprises a .cylinder 26 of plaster of Paris, or other suitable liquid-absorbent materialhaving the requisite strength, which is suported at its opposite ends by a plaster end wall 27 and a transparent end closure plate 28, respectively. j

The end wall 27 may be formed integrally with the cylinder 26, but it is usuallvmore convenient to manufacture them separately. It will Ybe observed that the end Y wall 27 has a cylindrical p .in a suitablefurnace. Y

3 A eripheral surface 27a that fits into the end of the mold 26, and an outwardly Yextending flange portion 27b that abuts'against the left Yend of the mold 26. l Y l,

VThe transparent end wall 28 may bernade of Lucite or glass and may havey an annular groove 23d receiving the end of the mold 26 for centering Vthe two with relation to each other. The groove 28a contains a rubber gasket 41 for effecting a seal. The plate 28 has a central aperture for receiving the arbor 13 and may be secured more or less permanently thereto by a pair of nuts 30 and 31, respectively. The end wall 27 also has a central aperture 4 of the casting is usually desirable, it is not essential in all instances, and the step involving the pour-casting of this end wall may sometimes be eliminated.

for receiving the arbor 13, and it maybe secured firmly 1 against the end of the mold 26 by a pressure plate 440 engaged by a nut 33 threaded on the arbor.

Brieliy, the operation of the apparatusdescribedV is as f follows. The hose connection V17 is connected Yto the, coupling 18 which may be prevented from rotating -withV Vthe arbor, as by an extension 18a thereon which engages Vsome stationary portion ofthe lathe, such as the base of the motor 14. Thelathe 10 is then v.energized to fdriv'e the chuck 11 at a suitable speed to` rotate the moldV assembly consisting of the arbor 13 and all of the parts secured thereto. The motor 14 is thenstarted to drive the' pump 1S, and a predetermined quantity of slip is poured into the funnel 1,6. Thisslipis delivered rapidly by the pump 15 through the hose 17 into the hollow arbor 13 and Hows from the arbor through longitudinal slits 13a therein within the mold assembly. The slits 13a Y maybe four in number, uniformly Ydistributed' circumlferentially around the arbor so that the slipfis delivered over substantially'the full length of the mold 2,6 at fourV Y circumferentially distributed areas thereof -and is thereafter quicklyA forced by the centrifugal Vforce into ja layer 35 against the inner surface of the mold 26. This body of slip 35 will hereinafter be referred to as the casting.

The spinning is continued while observingA the Vappearance of the casting 35 through the transparent end plate n23 until the casting takes onY certainl characteristics, whereupon the lathe is stopped, and thevhose line 17 is disconnected. The entire mold assembly is then removed from the lathe by disengaging the chuck 11 and the `tail stock 12 from the arbor 13. The Vnut 33 is then removed Vfrom the arbor, and the arbor and theltransparent plate 2,8 are removed as a unit from the mold 2,6 and the' end Y faces.

` stopped.

`a plug 35a inserted in the aperture infthe end Wall to Y close it. Thereafter, a quantity of slip is poured Vinto the mold suiicient to fill the re-entrant portion of' the end ywall, 2,7. The capillary attraction of the end wall 27 for the water in the slip causes a layer V36l of thickened slip -to buildup against the plaster. When this layer 36 Y has reachedY the desired thickness, fthe plug 35a is removed to Vdrain out the excess slip which has not set. The described technique for forming theV layer 36 is called pour-casting. The entire mold is then inverted, as shown in FIG. 4, and left in that position until dry.' The casting contracts away yfrom thev mold in the drying process, so'that at Y,the end thereof the mold can be lifted away from the casting. After furtheradrying, the casting is .then ready tobe tired Although the end wall portion 360i thejcasting is not essential in allV cases, itY is highly desirable'to' stiifen `the endofV the cas-ting and'maintain it in'true cylindrical reliation during -the'drying pr'ocessandV the firing process. After the casting has been'lired, the end wall. 36 isV cutV offby means ofa diamond saw` or othery Asuitable'equipnient, since it has no furtherV function; The open YendrofV the castingis `suliciently restrained against warping during As previously indicated, the changes in the casting 35 through the transparent end plate 2S. FIGS. 5 to 5c, inclusive, show successive stages of the spinning operation and the changes in the appearance of the end wall of the casting that lies against the transparent plate.

FIG. S shows the Vappearance very shortly after the slip is admitted to the mold. At first, the casting 35 appears homogeneous from its outer face lying against the mold 26 to its inner face and has the appearance of the slip, which in the cast of barium titanate is a light gray in color orrshading. YThe first change that appears is an outer zone of lighter, almost white, color 35a at the Contact of the casting lwith the mold. This is due t0 drying of the slip by movement of liquid therefrom into the mold 26. The zone 35a 'migrates inwardly, as shown in FIGS..5a and 5b. At the same time, an inner zone 3512 Vof substantially clear liquid appears at the inner surface. As previously explained, this results from the fact Vthat the solid particles of the slip are substantially heavier than the liquid and therefore migrate outwardly untilV they are tightly compacted together against the mold 26. The liquid, being of lesser weight, accumulates on the inner surface, because it cannot migrate through the solidparticles as rapidly as the latter are compacted.

n As shown in FIGS. 5a and 5b, the light zone 35a, where the casting is relatively dry, grows inwardly toward the inner surface as the spinning progresses. In FIG. 5b, the light zone 35a has reachedthe inner liquid zone 35b. The liquid of zone 35b, however, is constantly migrating through the zone 35a into the plaster mold, and eventually all liquid disappears, as shown in FIG. 5c, and the casting lhas a uniform appearance from its inner to the outer This is the stage at which the spinning must be if the spinning is stopped before the Water zone 351) disappears, thecasting will not have suflcient rigidity to support itself and will collapse shortly after the spinning is'stopped.

On the other hand, if the spinning is continued for any appreciable time after the water zone 3512 Vhas disappeared, the inner surface of the casting dries excessively .because of the centrifugal action urging the Water out through the solid residue. This causes cracks to begin in the inner surface and extend progressively outward toward the outer surface. Any such cracks render the casting useless, Abecause the cracks would cause failure in drying or in firing of the ceramic. f

Although observation of the end of the casting through the transparent window, as shown in FIGS. 5 to 5c, gives an excellent indication of the various stages through which the casting passes, the time at which the spinning should .be stopped can also be determined from observation of l as the liquid disappears, the inner the drying Yprocess. by the friction'between itand the.

the free inner surface of the casting, instead of observation of the end Vthat is in contact with the transparent window. Aslong as free liquid exists on the inner surface of the casting, it 'has a shiny appearance, whereas as soon surface has a dead or matte appearance. Y n

Various slip lcompositions are known the `trade for kuse in centrifugal casting and are lsuitable for the pres- 'v ent process'. VA slip Vthatrliasfbeen found satisfactory for use with the process described for the product-ion of barium titanate ceramic cylinders is as follows;

, Solids in powdered form Y grams 1000 1% solution tannic acid a ,cc-- 30 10% solution ammonium hydroxide cc- 30 Glycerine A. v 4. cc, l0 Distilled water cc 250 j; The solid component may consist'ofVbariumvtitanate alone, or barium titanate in combination with other titanates according to the properties that are desired in the red ceramic.

Although for the purpose of explaining the invention a particular embodiment thereof has 'been shown and de scribed, obvious modifications lwill occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

-I claim:

Ceramic casting apparatus comprising an arbor, means mounting -said arbor for rotation about a horizontal axis,

means for introducing ceramic slip into said apparatus yalong said arbor,

a mold body of hollow cylindrical shape including a porous inner surface,

a pair of end members mounted on said arbor and securing said mold body coaxial With the arbor and spaced therefrom,

the iirst one of said end members being transparent at least in the region adjacent to the inner surface of one end of said mold body to afford observation of the cross-section and inner surface of material centrifugally cast within said mold body,

the second of said end members including la portion internally recessed between the region of said mold body and said arbor,

the surface .ot recessed portion of said second end member being porous and constituting a molding surface for a reinforcing section of bodies cast Within said mold body,

said mold body and second end member being demountable from said arbor as la unit to allow removal of cylindrically cast bodies therein and positioning for pour-casting of a reinforcing section adjacent to said second end member.

References Cited in the le of this patent UNITED STATES PATENTS 

